Fabi ayyi aalaaa'i Rabbikumaa tukazzibaan

Word By Word:

Fabi ayyi: So which

alaa'i: (of the) favours

Rabbikuma: (of) your Lord

tukazzibaan: will ye both deny?

Translation:

Then which of the favors of your Lord will ye deny?

Ayah Tafseer:

The real basis of the crime according to the Quran is that the servant, who is benefiting by the blessings of his Sustainer, should harbor the misunderstanding that the blessings have been bestowed by no one, but have reached to him of their own accord, or that the blessings are not the gift of God but the fruit of his own ability or good fortune, or that they are the gift of God, but God has no right on His servants, or that God Himself has not done him those favors but some other being has gotten these done for him. These are the wrong concepts on account of which man becomes independent of God and being free from His obedience and service performs acts which He has forbidden, and avoids acts which He has enjoined. Seen in this light every crime and every sin is, in reality, a disavowal of the favors of Allah whether a person denies them by the word of mouth or not. But the person who, in fact, has no intention of the denial, but acknowledges the favors from the depths of his heart, commits an error occasionally because of human weakness, he shows repentance on it and tries to avoid it. This saves him from being included among the deniers. Apart from this, all other culprits are, in fact, beliers of Allah's blessings and deniers of His favors. That is why it has been said: When you will have been seized as culprits, then We shall see as to which of Our favors you deny. In Surah At-Takathur, Ayat 8, the same thing has been put thus: On that Day you will certainly be called to account for the blessings you had been granted. That is, it will be asked: Had We granted you these blessings or not. Then, what attitude did you adopt towards your Benefactor, and in what ways did you use His blessings.

Source: Yusuf Ali Translations and Tafheemul Quran by Abul A'la Maududi